Each line runs an equal distance either side of zero, with the arrow over a tick that may be positive or negative. Zero is always labelled and is never the answer - finding the middle of the line is not the same as reading it.
A number line is how negative numbers make sense before any arithmetic is done with them: -4 is not a strange kind of 4, it is a position four steps to the left of zero. Students who meet integers this way find -8 < -3 obvious rather than backwards.
Because it is further left on the line. That is the whole answer, and it is why integers are introduced on a number line - the rule looks arbitrary written down and is obvious drawn.
Neither. It is the origin the line is built around, which is why it is always labelled here and never the marked value.
Sixth grade, where negative numbers are introduced and ordered. Adding and subtracting them is seventh, and our integer worksheets cover that.
Symmetrically either side of zero, up to half the Largest number setting in each direction, so the line stays the same length as a whole-number one.